Interconnect structure and method

ABSTRACT

A semiconductor device comprises a first semiconductor chip including a first substrate and a plurality of first metal lines formed over the first substrate and a second semiconductor chip bonded on the first semiconductor chip, wherein the second semiconductor chip comprises a second substrate and a plurality of second metal lines formed over the second substrate. The semiconductor device further comprises a conductive plug coupled between the first metal lines and the second metal lines, wherein the conductive plug comprises a first portion formed over a first side of a hard mask layer, wherein the first portion is of a first width and a second portion formed over a second side of the hard mask layer, wherein the second portion is of a second width greater than or equal to the first width.

This application claims the benefit of U.S. Provisional Application Ser. No. 61/777,870, filed on Mar. 12, 2013, entitled “Interconnect Structure and Method,” which application is hereby incorporated herein by reference in its entirety.

BACKGROUND

The semiconductor industry has experienced rapid growth due to continuous improvements in the integration density of a variety of electronic components (e.g., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from repeated reductions in minimum feature size (e.g., shrink the semiconductor process node towards the sub-20 nm node), which allows more components to be integrated into a given area. As the demand for miniaturization, higher speed and greater bandwidth, as well as lower power consumption and latency has grown recently, there has grown a need for smaller and more creative packaging techniques of semiconductor dies.

As semiconductor technologies further advance, stacked semiconductor devices have emerged as an effective alternative to further reduce the physical size of a semiconductor device. In a stacked semiconductor device, active circuits such as logic, memory, processor circuits and the like are fabricated on different semiconductor wafers. Two or more semiconductor wafers may be installed on top of one another to further reduce the form factor of the semiconductor device.

Two semiconductor wafers may be bonded together through suitable bonding techniques. The commonly used bonding techniques include direct bonding, chemically activated bonding, plasma activated bonding, anodic bonding, eutectic bonding, glass frit bonding, adhesive bonding, thermo-compressive bonding, reactive bonding and/or the like. Once two semiconductor wafers are bonded together, the interface between two semiconductor wafers may provide an electrically conductive path between the stacked semiconductor wafers.

One advantageous feature of stacked semiconductor devices is much higher density can be achieved by employing stacked semiconductor devices. Furthermore, stacked semiconductor devices can achieve smaller form factors, cost-effectiveness, increased performance and lower power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a cross sectional view of a stacked semiconductor device prior to a bonding process in accordance with various embodiments of the present disclosure;

FIG. 2 illustrates a cross sectional view of the semiconductor device shown in FIG. 1 after a bottom anti-reflection coating (BARC) layer is formed over the first semiconductor wafer and a patterning process is applied to the substrate of the first semiconductor wafer in accordance with various embodiments of the present disclosure;

FIG. 3 illustrates a cross section view of the semiconductor device shown in FIG. 2 after a dielectric layer is deposited over the semiconductor device in accordance with various embodiments of the present disclosure;

FIG. 4 illustrates a cross section view of the semiconductor device shown in FIG. 3 after a mask layer is formed over the semiconductor device in accordance with various embodiments of the present disclosure;

FIG. 5 illustrates a cross section view of the semiconductor device shown in FIG. 4 after an etching process is applied to the semiconductor device in accordance with various embodiments of the present disclosure;

FIG. 6 illustrates a cross sectional view of the semiconductor device shown in FIG. 5 after the remaining photoresist layer has been removed in accordance with various embodiments of the present disclosure;

FIG. 7 illustrates a cross sectional view of the semiconductor device shown in FIG. 6 after a conductive material has been filled in the openings in accordance with various embodiments of the present disclosure;

FIG. 8 illustrates a cross section view of the semiconductor device shown in FIG. 7 after a chemical mechanical polish (CMP) process is applied to the top surface of the semiconductor device in accordance with various embodiments of the present disclosure;

FIG. 9 illustrates a cross sectional view of the semiconductor device shown in FIG. 8 after a dielectric layer is formed on the semiconductor device in accordance with various embodiments of the present disclosure;

FIG. 10 illustrates a cross sectional view of another stacked semiconductor device in accordance with various embodiments of the present disclosure;

FIG. 11 illustrates a cross sectional view of yet another stacked semiconductor device in accordance with various embodiments of the present disclosure;

FIG. 12 illustrates a cross sectional view of a backside illuminated imager sensor including a stacked wafer structure in accordance with various embodiments of the present disclosure;

FIG. 13 illustrates a top view of the hard mask in accordance with various embodiments of the present disclosure; and

FIG. 14 illustrates another top view of the hard mask in accordance with various embodiments of the present disclosure.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferred embodiments in a specific context, a method for forming interconnect structures for a stacked semiconductor device. The invention may also be applied, however, to a variety of semiconductor devices. Hereinafter, various embodiments will be explained in detail with reference to the accompanying drawings.

FIG. 1 illustrates a cross sectional view of a stacked semiconductor device prior to a bonding process in accordance with various embodiments of the present disclosure. Both the first semiconductor wafer 110 and the second semiconductor wafer 210 include a semiconductor substrate (e.g., first substrate 102 and second substrate 202) and a plurality of interconnect structures (e.g., metal lines 106, 108, 206 and 208) formed over the semiconductor substrate. The first semiconductor wafer 110 is used as an example to illustrate the detailed structure of the semiconductor wafers prior to a bonding process.

As shown in FIG. 1, the first semiconductor wafer 110 may comprises a first substrate 102 and a plurality of inter-metal dielectric layers 104 formed over the first substrate 102. In addition, a plurality of metal lines such as metal lines 106 and 108 are formed in the inter-metal dielectric layers 104.

The first substrate 102 may be formed of silicon, although it may also be formed of other group III, group IV, and/or group V elements, such as silicon, germanium, gallium, arsenic, and combinations thereof. The first substrate 102 may also be in the form of silicon-on-insulator (SOI). The SOI substrate may comprise a layer of a semiconductor material (e.g., silicon, germanium and/or the like) formed over an insulator layer (e.g., buried oxide and/or the like), which is formed in a silicon substrate. In addition, other substrates that may be used include multi-layered substrates, gradient substrates, hybrid orientation substrates, any combinations thereof and/or the like.

The first substrate 102 may further comprise a variety of electrical circuits (not shown). The electrical circuits formed on the first substrate 102 may be any type of circuitry suitable for a particular application. In accordance with some embodiments, the electrical circuits may include various n-type metal-oxide semiconductor (NMOS) and/or p-type metal-oxide semiconductor (PMOS) devices such as transistors, capacitors, resistors, diodes, photo-diodes, fuses and/or the like.

The electrical circuits may be interconnected to perform one or more functions. The functions may include memory structures, processing structures, sensors, amplifiers, power distribution, input/output circuitry and/or the like. One of ordinary skill in the art will appreciate that the above examples are provided for illustrative purposes only and are not intended to limit the various embodiments to any particular applications.

The inter-metal dielectric layers 104 are formed over the first substrate 102. As shown in FIG. 1, the inter-metal dielectric layers 104 may comprise a plurality of metal lines such as metal lines 106 and 108.

The metal lines 106 and 108 may be made through any suitable formation process (e.g., lithography with etching, damascene, dual damascene, or the like) and may be formed using suitable conductive materials such as copper, aluminum, aluminum alloys, copper alloys or the like.

As shown in FIG. 1, the first semiconductor wafer 110 will be stacked on top of the second semiconductor wafer 210. In some embodiments, a plurality of bonding pads are formed in the first semiconductor wafer 110 and the second semiconductor wafer 210 respectively. Furthermore, the bonding pads located at the second semiconductor wafer 210 are aligned face-to-face with their corresponding bonding pads located at the first semiconductor wafer 110. The first semiconductor wafer 110 and the second semiconductor wafer 210 are bonded together through suitable bonding techniques such as direct bonding.

In accordance with some embodiments, in a direct bonding process, the connection between the first semiconductor wafer 110 and the second semiconductor wafer 210 can be implemented through metal-to-metal bonding (e.g., copper-to-copper bonding), dielectric-to-dielectric bonding (e.g., oxide-to-oxide bonding), metal-to-dielectric bonding (e.g., oxide-to-copper bonding), any combinations thereof and/or the like.

FIG. 2 illustrates a cross sectional view of the semiconductor device shown in FIG. 1 after a bottom anti-reflection coating (BARC) layer is formed over the first semiconductor wafer and a patterning process is applied to the substrate of the first semiconductor wafer in accordance with various embodiments of the present disclosure. The BARC layer 112 is formed on the backside of the first substrate 102. Throughout the description, the side of the first substrate 102 adjacent to the BARC layer 112 is referred to the backside of the first substrate 102.

The BARC layer 112 may be formed of a nitride material, an organic material, an oxide material and the like. The BARC layer 112 may be formed using suitable techniques such as chemical vapor deposition (CVD) and/or the like.

A patterned mask such as a photoresist mask and/or the like may be formed over the BARC layer 112 using suitable deposition and photolithography techniques. A suitable etching process, such as a reactive ion etch (RIE) or other dry etch, an anisotropic wet etch, or any other suitable anisotropic etch or patterning process may be applied to the first substrate 102 of the first semiconductor wafer 110. As a result, a plurality of openings 114 and 116 are formed in the first substrate 102.

FIG. 3 illustrates a cross section view of the semiconductor device shown in FIG. 2 after a dielectric layer is deposited over the semiconductor device in accordance with various embodiments of the present disclosure. As shown in FIG. 3, a dielectric layer 302 is formed over the bottoms and sidewalls of the openings 114 and 116. In addition, the dielectric layer 302 is formed over the BARC layer 112.

The dielectric layer 302 may be formed of various dielectric materials commonly used in integrated circuit fabrication. For example, the dielectric layer 302 may be formed of silicon dioxide, silicon nitride or a doped glass layer such as boron silicate glass and the like. Alternatively, dielectric layer may be a layer of silicon nitride, a silicon oxynitride layer, a polyamide layer, a low dielectric constant insulator or the like. In addition, a combination of the foregoing dielectric materials may also be used to form the dielectric layer 302. In accordance with some embodiments, the dielectric layer 302 may be formed using suitable techniques such as sputtering, oxidation, CVD and/or the like.

FIG. 4 illustrates a cross section view of the semiconductor device shown in FIG. 3 after a mask layer is formed over the semiconductor device in accordance with various embodiments of the present disclosure. A patterned mask 402 is formed over the sidewalls of the openings 114 and 116 (shown in FIG. 3). As shown in FIG. 4, two new openings 404 and 406 are formed after the patterned mask 402 are formed along the sidewalls of the openings 114 and 116.

The patterned mask 402 may be a photoresist layer. The patterned mask 402 is formed on the top surface of the semiconductor device using suitable deposition and photolithography techniques.

FIG. 5 illustrates a cross section view of the semiconductor device shown in FIG. 4 after an etching process is applied to the semiconductor device in accordance with various embodiments of the present disclosure. A suitable etching process, such as a dry etch, an anisotropic wet etch, or any other suitable anisotropic etch or patterning process, may be performed to form openings 504 and 506. The openings 504 and 506 are respective extensions of the openings 404 and 406. In particular, the openings 504 and 506 extend through the inter-metal dielectric layers 104 and 204 as well as the bonding interface of two stacked wafers. As shown in FIG. 5, the metal lines 106, 108, 206 and 208 are exposed after the openings 504 and 506 have been formed.

It should be noted that the metal lines 106 and 108 are formed of suitable metal materials such as copper, which is of a different etching rate (selectivity) from the first substrate 102 and the inter-metal dielectric layers. As such, the metal lines 106 and 108 may function as a hard mask layer for the etching process of the inter-metal dielectric layers 104 and 204. A selective etching process may be employed to etch the inter-metal dielectric layers 104 and 204 rapidly while etching only a portion of the metal lines 106 and 108. As shown in FIG. 5, the exposed portion of the hard mask layer (e.g., metal lines 106 and 108) is partially etched away, thereby forming a recess such as recess 502. The depth of the recess 502 may vary depending on different applications and design needs.

FIG. 6 illustrates a cross sectional view of the semiconductor device shown in FIG. 5 after the remaining photoresist layer has been removed in accordance with various embodiments of the present disclosure. The remaining photoresist layer shown in FIG. 5 may be removed by using suitable photoresist stripping techniques such as chemical solvent cleaning, plasma ashing, dry stripping and/or the like. The photoresist stripping techniques are well known and hence are not discussed in further detail herein to avoid repetition.

FIG. 7 illustrates a cross sectional view of the semiconductor device shown in FIG. 6 after a conductive material has been filled in the openings in accordance with various embodiments of the present disclosure. In some embodiments, a barrier layer and a seed layer may be deposited prior to a plating process, through which the conductive material is filled into the openings.

A barrier layer 710 may be deposited on the bottom as well as the sidewalls of the opening (e.g., opening 404 shown in FIG. 6). The barrier layer 710 may be formed of titanium, titanium nitride, tantalum, tantalum nitride, and combinations thereof and/or the like. In some embodiments, the barrier layer 710 may be uniform in thickness. In alternative embodiments, the barrier layer 710 may be non-uniform in thickness. The barrier layer 710 may be formed using suitable fabrication techniques such as atomic layer deposition (ALD), plasma enhance CVD (PECVD), plasma enhanced physical vapor deposition (PEPVD) and/or the like.

In addition, a seed layer (not shown) may be deposited over the barrier layer 710. The seed layer may be may be formed of copper, nickel, gold, any combination thereof and/or the like. The seed layer may be formed by suitable deposition techniques such as PVD, CVD and/or the like.

Moreover, the seed layer may be alloyed with a material that improves the adhesive properties of the seed layer so that it can act as an adhesion layer. For example, the seed layer may be alloyed with a material such as manganese or aluminum, which will migrate to the interface between the seed layer and the barrier layer 710 and will enhance the adhesion between these two layers. The alloying material may be introduced during formation of the seed layer. The alloying material may comprise no more than about 10% of the seed layer.

Once the barrier layer 710 and the seed layer has been deposited in the openings, a conductive material, which includes tungsten, titanium, aluminum, copper, any combinations thereof and/or the like, is filled into the openings, forming conductive plugs 702 and 704. In some embodiments, the conductive material may be filled in the openings through an electroplating process.

FIG. 8 illustrates a cross section view of the semiconductor device shown in FIG. 7 after a chemical mechanical polish (CMP) process is applied to the top surface of the semiconductor device in accordance with various embodiments of the present disclosure. A planarization process, such as CMP, etch back step and the like, may be performed to planarize the top surface of the semiconductor device. As shown in FIG. 8, a portion of the conductive material has been removed as a result. As shown in FIG. 8, there may be two conductive plugs 802 and 804 formed in the semiconductor device after the CMP process is performed on the semiconductor device.

As shown in FIG. 8, each conductive plug (e.g., conductive plugs 802 and 804) may comprise three portions. A first portion is from the metal line 206 to the hard mask layer formed by the metal lines 106 and 108. The first portion is of a width W1 as shown in FIG. 8. A second portion is from the hard mask layer to the front side of the first substrate 102. The second portion is of a width W2 as shown in FIG. 8. A third portion is from the backside of the first substrate 102 to the backside of the first substrate 102. The third portion is of a width W3 as shown in FIG. 8. In some embodiments, W2 is greater than or equal to W1. W3 is greater than W2.

FIG. 9 illustrates a cross sectional view of the semiconductor device shown in FIG. 8 after a dielectric layer is formed on the semiconductor device in accordance with various embodiments of the present disclosure. The dielectric layer 902 may comprise commonly used dielectric materials, such as silicon nitride, silicon oxynitride, silicon oxycarbide, silicon carbide, combinations thereof, and multi-layers thereof. The dielectric layer 902 may be deposited over the semiconductor device through suitable deposition techniques such as sputtering, CVD and the like.

The conductive plugs (e.g., conductive plug 802) include three portions as described above with respect to FIG. 8. The portion from the hard mask layer (e.g., metal line 106) to the metal line 206 may be alternatively referred to as a three-dimensional structure 904 throughout the description.

One advantageous feature of the stacked wafer having the conductive plugs 802 and 804 shown in FIG. 9 is that the active circuits of both semiconductor wafers are connected to each other through a single conductive plug (e.g., conductive plug 802). Such a single conductive plug helps to further reduce form factor. Furthermore, in comparison to stacked semiconductor devices connected by multiple conductive plugs, the single conductive plug coupled between two semiconductor wafers shown in FIG. 9 helps to cut power consumption and prevent parasitic interference.

It should be noted while FIG. 9 illustrates two semiconductor wafers stacked together, one skilled in the art will recognize that the stacked semiconductor device shown in FIG. 9 is merely an example. There may be many alternatives, variations and modifications. For example, the stacked semiconductor device may accommodate more than two semiconductor wafers.

FIG. 10 illustrates a cross sectional view of another stacked semiconductor device in accordance with various embodiments of the present disclosure. The stacked semiconductor device 1000 is similar to the stacked semiconductor device 100 shown in FIG. 9 except that the hard mask layer is formed by contacts, which is located adjacent to the interface between the first substrate 102 and the inter-metal dielectric layers 104.

The contacts may be formed in an inter-layer dielectric layer (not shown). The inter-layer dielectric layer may comprise a material such as boron phosphorous silicate glass (BPSG), although any suitable dielectrics may be used for either layer. The inter-layer dielectric layer may be formed using a process such as PECVD, although other processes may alternatively be used.

The contacts 1006 and 1008 may be formed through the inter-layer dielectric layer with suitable photolithography and etching techniques. Generally, these photolithography techniques involve depositing a photoresist material, which is masked, exposed, and developed to expose portions of the inter-layer dielectric layer that are to be removed. The remaining photoresist material protects the underlying material from subsequent processing steps, such as etching.

The contacts 1006 and 1008 may comprise a barrier/adhesion layer (not shown) to prevent diffusion and provide better adhesion for the contacts 1006 and 1008. In some embodiments, the contacts 1006 and 1008 may be formed of any suitable conductive material, such as a highly-conductive, low-resistive metal, elemental metal, transition metal, or the like. In accordance with an embodiment, the contacts 1006 and 1008 are formed of tungsten, although other materials, such as copper, aluminum and/or the like, could alternatively be utilized. In an embodiment in which the contacts 1006 and 1008 are formed of tungsten, the contacts 1006 and 1008 may be deposited by CVD techniques known in the art, although any method of formation could alternatively be used.

As shown in FIG. 11, the conductive plugs (e.g., conductive plugs 1002 and 1004) include three portions. The portion from the hard mask layer (e.g., contact 1006) to the metal line 206 may be alternatively referred to as a three-dimensional structure 1003 throughout the description.

FIG. 11 illustrates a cross sectional view of yet another stacked semiconductor device in accordance with various embodiments of the present disclosure. The stacked semiconductor device 1100 is similar to the stacked semiconductor device 100 shown in FIG. 9 except that the etching hard mask is formed by redistribution layers, which are located adjacent to the interface of two semiconductor wafers.

The redistribution metal lines 1106 and 1108 may be a single material layer, or a multi-layered structure and may be made of metals such as titanium, titanium nitride, aluminum, tantalum, copper and combinations thereof. The redistribution metal line 1106 and 1108 may be made by any suitable method known in the art such as physical vapor deposition (PVD), sputter, CVD, electroplating and/or the like.

The conductive plugs (e.g., conductive plugs 1102 and 1104) include three portions. The portion from the hard mask layer (e.g., redistribution metal lines 1106 and 1108) to the metal line 206 may be alternatively referred to as a three-dimensional structure 1103 throughout the description.

It should be noted that the first semiconductor wafer 110 may be bonded on the second semiconductor wafer 210 through a suitable metal-dielectric bonding technique such as a copper-silicon oxide nitride (Cu—SiON) bonding process.

It should further be noted while FIG. 9, FIG. 10 and FIG. 11 illustrate hard mask layers formed by metal lines, contacts and redistribution lines respectively, one skilled in the art will recognize that hard mask layers shown in FIGS. 9-11 are merely examples. There may be many alternatives, variations and modifications. For example, the hard mask layer may be formed by a plurality of isolation regions, poly-silicon regions, any combinations thereof and/or the like.

FIG. 12 illustrates a cross sectional view of a backside illuminated imager sensor including a stacked wafer structure in accordance with various embodiments of the present disclosure. The backside illuminated image sensor 1200 comprises two semiconductor wafers, namely a sensor wafer 1201 and an application-specific integrated circuit (ASIC) wafer 1203. As shown in FIG. 12, the sensor wafer 1201 is stacked on top of the ASIC 1203. In some embodiments, the sensor wafer 1201 and the ASIC wafer 1203 are connected to each other through suitable three-dimensional structures such as the three-dimensional structure 904 shown in FIG. 9, the three-dimensional structure 1003 shown in FIG. 10, the three-dimensional structure 1103 shown in FIG. 11 and any combinations thereof.

The ASIC wafer 1203 may comprise a plurality of logic circuits such as logic circuits 1206 and 1208. In some embodiments, the logic circuits may be an analog-to-digital converter. However, the logic circuits may be other functional circuits that may be utilized within a backside illuminated image sensor. For example, the logic circuits 1206 and 1208 may be a data processing circuit, a memory circuit, a bias circuit, a reference circuit, any combinations thereof and/or the like.

The ASIC wafer 1203 may further comprise a plurality of interconnection layers and a plurality of metal lines 1220, 1222, 1224 and 1226 embedded in the interconnection layers. The metal lines 1220, 1222, 1224 and 1226 may function as interconnection structures. As indicated by the arrows shown in FIG. 12, the metal lines 1220, 1222, 1224 and 1226 provide signal paths between logic circuits 1206 and 1208, and the sensor wafer 1201.

The metal lines 1220, 1222, 1224 and 1226 may be made through any suitable formation process (e.g., lithography with etching, damascene, dual damascene, or the like) and may be formed using suitable conductive materials such as copper, aluminum, aluminum alloys, copper alloys or the like.

The sensor wafer 1201 is fabricated by CMOS process techniques known in the art. In particular, the sensor wafer 1201 comprises an epitaxial layer over a silicon substrate. According to the fabrication process of backside illuminated image sensors, the silicon substrate has been removed in a backside thinning process until the epitaxial layer is exposed. A portion of epitaxial layer may remain. A p-type photo active region and an n-type photo active region (not shown respectively) are formed in the remaining epitaxial layer.

The photo active regions such as the p-type photo active region and the n-type photo active region may form a PN junction, which functions as a photodiode. As shown in FIG. 12, the imager sensor 1110 may comprise a plurality of photodiodes.

The sensor wafer 1201 may comprise a transistor (not shown). In particular, the transistor may generate a signal related to the intensity or brightness of light that impinges on the photo active regions. In accordance with an embodiment, the transistor may be a transfer transistor. However, the transistor may be an example of the many types of functional transistors that may be utilized within a backside illuminated image sensor. For example, the transistor may include other transistors located within a backside illuminated image sensor, such as a reset transistor, a source follower transistor or a select transistor. All suitable transistors and configurations that may be utilized in an image sensor are fully intended to be included within the scope of the embodiments.

The sensor wafer 1201 may comprise a plurality of interconnection layers and metal lines embedded in the interconnection layers. The metal lines 1120, 1122, 1124 and 1126 may provide signal paths between the sensor wafer 1201 and the ASIC wafer 1203. In particular, as indicated by the arrows shown in FIG. 12, an external signal may enter the backside illuminated image sensor 1200 through the aluminum copper pad 1112, and then reach the metal routing (e.g., metal line 1120) through interconnect structures such through vias (not shown). The external signal may further pass through a three-dimensional structure 1210. The three-dimensional structure 1210 may be the three-dimensional structure 904 shown in FIG. 9, the three-dimensional structure 1003 shown in FIG. 10, the three-dimensional structure 1103 shown in FIG. 11 and/or any combinations thereof.

After the external signal passes the three-dimensional structure 1210, the external signal may reach the logic circuit 1206 through the metal routing (e.g., metal line 1220) of the ASIC wafer 1203.

When a signal leaves the logic circuit 1206, it reaches the image sensor 1110 through a conductive path formed by the metal routing (e.g., metal line 1222) of the ASIC wafer 1203, the three-dimensional structure 1210, the metal routing (e.g., metal line 1122) of the sensor wafer 1201.

After the image sensor 1110 generates a signal, the signal is sent to the logic circuit 1208 through a path formed by the metal routing (e.g., metal line 1124) of the sensor wafer 1201, the three-dimensional structure 1210, the metal routing (e.g., metal line 1224) of the ASIC wafer 1203. Furthermore, the signal may be sent outside of the backside illuminated image sensor 1200 from the logic circuit 1208 through a path formed by the metal routing (e.g., metal line 1226) of the ASIC wafer 1203, the three-dimensional structure 1210, the metal routing (e.g., metal line 1126) of the sensor wafer 1201 and the aluminum copper pad 1114.

The logic circuit 1206 and 1208 may be coupled to aluminum copper pads 1112 and 1114. As shown in FIG. 12, the aluminum copper pads 1112 and 1114 may be formed on the backside of the sensor wafer 1201.

It should be noted that the location of the aluminum copper pads 1112 and 1114 shown in FIG. 12 is merely an example. A person skilled in the art will recognize that there may be many alternatives, modifications and variations. For example, the aluminum copper pads 1112 and 1114 may be formed on the non-bonding side of the ASIC wafer 1203. The form factor of a backside illuminated image sensor can be reduced by forming the aluminum copper pads 1112 and 1114 on the non-bonding side of the ASIC wafer 1203.

One advantageous feature of having input/output terminals formed on the non-bonding side of the ASIC wafer 1203 is that the density as well as quantum efficiency of the backside illuminated image sensor 1200 can be improved as a result.

FIG. 13 illustrates a top view of the hard mask in accordance with various embodiments of the present disclosure. As described above with respect to FIG. 9, FIG. 10 and FIG. 11, the hard mask layers may be formed by metal lines, contacts and redistribution lines respectively. While the cross sectional view 1301 shows the hard mask layer includes two portions (e.g., metal lines 106 and 108), these two portions may be from a continuous ring shaped region as illustrated by the top view 1302. The top view 1302 of the hard mask layer shows the hard mask layer is of a ring shape. The inside diameter of the ring shaped hard mask layer is denoted as W1.

It should be noted that the internal circle of the ring shaped hard mask layer can be replaced by other suitable shapes such as a square as shown by the top view 1304. It is within the scope and spirit of various embodiments of the present disclosure that the top view of the hard mask layer may comprise other shapes, such as, but no limited to oval, triangular, polygonal and/or the like.

FIG. 14 illustrates another top view of the hard mask in accordance with various embodiments of the present disclosure. The top views of FIG. 14 are similar to those shown in FIG. 13 except that the ring shape is replaced by a square with an opening. The top view 1402 shows a square with a square shaped opening. The top view 1404 shows a square with a circular shaped opening.

In accordance with an embodiment, an apparatus comprises a first semiconductor chip including a first substrate and a plurality of first metal lines formed over the first substrate and a second semiconductor chip bonded on the first semiconductor chip, wherein the second semiconductor chip comprises a second substrate and a plurality of second metal lines formed over the second substrate.

The semiconductor device further comprises a conductive plug coupled between the first metal lines and the second metal lines, wherein the conductive plug comprises a first portion formed over a first side of a hard mask layer, wherein the first portion is of a first width and a second portion formed over a second side of the hard mask layer, wherein the second portion is of a second width greater than or equal to the first width.

In accordance with an embodiment, a device comprises a first chip, a second chip and a conductive plug. The first chip comprises a first substrate and a plurality of first interconnect components formed in first inter-metal dielectric layers and over the first substrate. The second chip is bonded on the first chip. The second chip comprises a second substrate and a plurality of second interconnect components formed in second inter-metal dielectric layers and over the second substrate. The conductive plug is formed through the first substrate and the first inter-metal dielectric layers and formed partially through the second inter-metal dielectric layers. The conductive plug is coupled between the first interconnect components and the second interconnect components.

In accordance with an embodiment, a method comprises bonding a first semiconductor wafer on a second semiconductor wafer, wherein the first semiconductor wafer comprises a first substrate, first inter-metal dielectric layers and first interconnect structures formed in the first inter-metal dielectric layers and over the first substrate and the second semiconductor wafer comprises a second substrate, second inter-metal dielectric layers and second interconnect structures formed in the second inter-metal dielectric layers and over the second substrate.

The method further comprises patterning the first substrate to form a first opening through the first substrate, forming a second opening using an etching process and using the first interconnect structures as a hard mask layer, wherein the second opening is formed through the first inter-metal dielectric layers and partially through the second inter-metal dielectric layers and plating a conductive material in the first opening and the second opening.

Although embodiments of the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. An apparatus comprising: a first semiconductor chip including a first substrate and a plurality of first metal lines formed over the first substrate; a second semiconductor chip bonded on the first semiconductor chip, wherein the second semiconductor chip comprises a second substrate and a plurality of second metal lines formed over the second substrate; and a conductive plug coupled between the first metal lines and the second metal lines, wherein the conductive plug comprises: a first portion formed over a first side of a hard mask layer, wherein the first portion is of a first width; a second portion formed over a second side of the hard mask layer and below the first substrate, wherein the second portion is of a second width greater than or equal to the first width; and a third portion formed in the first substrate, wherein the third portion is of a third width greater than the second width.
 2. The apparatus of claim 1, wherein: the hard mask layer is formed by the first metal lines.
 3. The apparatus of claim 1, wherein: the hard mask layer is formed by redistribution lines of the first semiconductor chip.
 4. The apparatus of claim 1, wherein: the hard mask layer is formed by contacts of the first semiconductor chip.
 5. The apparatus of claim 1, wherein: the first portion is located between the first metal lines and the second metal lines; and the second portion is located between the first metal lines and a backside of the first substrate.
 6. The apparatus of claim 1, wherein: the first portion is located between contacts of the first semiconductor chip and the second metal lines; and the second portion is located between the contacts of the first semiconductor chip and a backside of the first substrate.
 7. The apparatus of claim 1, wherein: the first portion is located between redistribution lines of the first semiconductor chip and the second metal lines; and the second portion is located between the redistribution lines of the first semiconductor chip and a backside of the first substrate.
 8. A device comprising: a first chip comprising: a first substrate; and a plurality of first interconnect components formed in first inter-metal dielectric layers and over the first substrate; a second chip bonded on the first chip, wherein the second chip comprises: a second substrate; and a plurality of second interconnect components formed in second inter-metal dielectric layers and over the second substrate; and a conductive plug formed through the first substrate and the first inter-metal dielectric layers and formed partially through the second inter-metal dielectric layers, wherein the conductive plug is coupled between the first interconnect components and the second interconnect components, and the first interconnect in the first inter-metal dielectric comprises a hard mask wherein the width of the conductive plug above the hard mask is greater than the width below the hard mask and the width of the conductive plug in the substrate is greater than the width of the conductive plug above hard mask.
 9. The device of claim 8, further comprising: a hard mask layer formed in the first chip, wherein the hard mask layer divides the conductive plug into a first portion and a second portion, wherein: the first portion is adjacent to the first substrate; and the first portion is of a width greater than or equal to a width of the second portion.
 10. The device of claim 9, wherein: the hard mask layer is formed by two first interconnect components.
 11. The device of claim 9, wherein: the hard mask layer is formed by two contacts of the first chip.
 12. The device of claim 9, wherein: the hard mask layer is formed by two redistribution lines of the first chip.
 13. The device of claim 8, wherein: the conductive plug is formed of copper.
 14. A method comprising: bonding a first semiconductor wafer on a second semiconductor wafer, wherein: the first semiconductor wafer comprises a first substrate, first inter-metal dielectric layers and first interconnect structures formed in the first inter-metal dielectric layers and over the first substrate; and the second semiconductor wafer comprises a second substrate, second inter-metal dielectric layers and second interconnect structures formed in the second inter-metal dielectric layers and over the second substrate; patterning the first substrate to form a first opening in the first substrate; forming a second opening using an etching process and using the first interconnect structures as a hard mask layer, wherein the second opening is formed through the first inter-metal dielectric layers and partially through the second inter-metal dielectric layers; and plating a conductive material in the first opening and the second opening to form a conductive plug, the first interconnect in the first inter-metal dielectric comprises a hard mask wherein the width of the conductive plug above the hard mask is greater than the width below the hard mask and the width of the conductive plug in the substrate is greater than the width of the conductive plug above hard mask greater than a width of a portion of the conductive plug in the first inter-metal dielectric layers.
 15. The method of claim 14, further comprising: depositing a bottom anti-reflection coating layer on a backside of the first semiconductor wafer; and removing a portion of the first substrate to form the first opening.
 16. The method of claim 14, further comprising: forming the second opening using a plurality of metal lines of the first semiconductor wafer as the hard mask layer.
 17. The method of claim 14, further comprising: forming the second opening using a plurality of contacts of the first semiconductor wafer as the hard mask layer.
 18. The method of claim 14, further comprising: forming the second opening using a plurality of redistribution lines of the first semiconductor wafer as the hard mask layer.
 19. The method of claim 14, wherein: the conductive material is copper.
 20. The method of claim 14, further comprising: after the step of plating the conductive material in the first opening and the second opening, applying a chemical mechanical polish process to a backside of the first semiconductor wafer, and depositing a dielectric layer over the backside of the first semiconductor wafer through a chemical vapor deposition process. 